1. Field of the Invention
The present invention relates to an object information acquisition apparatus. In particular, the present invention relates to an object information acquisition apparatus that acquires acoustic waves generated by irradiating an object with light.
2. Description of the Related Art
Imaging techniques for irradiating an object such as a living body with light from a light source such as a laser and for imaging information in the living body resulting from the incident light have been discussed. Among such techniques is a photoacoustic imaging technique.
The photoacoustic imaging technique includes irradiating the living body with pulsed light emitted from the light source, receiving acoustic waves that are generated by body tissue absorbing the light propagated and diffused through the living body, and performing analysis processing on the received acoustic waves to visualize information in the living body, which is an object. The results are optical characteristic distributions in the living body, or an optical absorption coefficient distribution and an oxygen saturation distribution, in particular. Studies have been made to diagnose the object by using an image of the optical absorption coefficient distribution and such.
In photoacoustic imaging, an initial sound pressure P0 of the acoustic waves occurring from a light absorber in the object can be expressed by the following equation (1) (see Japanese Patent Application Laid-Open No. 2009-18153):P0=Γ·μa·Φ  (1)where Γ is a Grüneisen coefficient. The Grüneisen coefficient Γ is a product of a coefficient of volumetric expansion β and the square of a speed of sound c, divided by a specific heat at constant pressure CP. Given the object, Γ is known to have a near constant value. μa is an optical absorption coefficient of the light absorber and Φ is a fluence in a regional area.
A sound pressure P, which is the magnitude of the acoustic waves propagated through the object, is measured for temporal changes, and an initial sound pressure distribution is calculated from the measurements. The calculated initial sound pressure distribution is divided by the Grüneisen coefficient Γ to determine a light energy absorption density distribution, which is the product of μa and Φ. As shown by equation (1), in order to determine the distribution of the optical absorption coefficient μa from the distribution of the initial sound pressure P0, the distribution of the fluence Φ (light amount distribution) in the object needs to be determined.
The distribution of the fluence Φ can be calculated by using a relative light irradiation density distribution (hereinafter, also referred to as a “relative illuminance distribution”) of light with which a surface is irradiated. The relative illuminance distribution is a relative light intensity distribution in a light irradiation area of the surface of the object. The relative illuminance distribution is determined by capturing an optical pattern that occurs on the object surface when the object surface is irradiated with the light. The relative illuminance distribution can be analyzed to calculate the light amount distribution in the object. Using the light amount distribution, the optical absorption distribution in the object can be determined from equation (1).
An apparatus using the photoacoustic imaging technique is intended to measure a living body as the object. More specifically, intended objects include diagnostic target regions such as a breast, fingers, hands, and feet of a human body or an animal. The apparatus generates image data on the optical absorption distribution in such target regions.
For measurement, a fixed target region is irradiated with light pulses to acquire acoustic waves from inside the target region. To determine the relative illuminance distribution of the light with which the surface is irradiated, the object may be sandwiched between two holding plates serving as a holding member, for example. The irradiation of the light pulses and the acquisition of the acoustic waves occurring in the object may be performed through the holding plates.
At least either one of the two holding plates may be configured to be movable in the direction of sandwiching the object, whereby the object-holding operation can be smoothly performed. An optical pattern of the light pulses on the holding plate can be captured to determine the relative illuminance distribution of the light with which the object surface is irradiated.
If the holding plate moves in the direction of sandwiching the object and the light emission position is fixed, the distance from the light emission position to the object changes with a change in the distance where the object is held. Since the total amount of light incident on the object varies with the distance from the emission position, the relative illuminance distribution occurring on the object surface changes. To obtain an accurate optical absorption distribution, the relative illuminance distribution occurring on the object surface needs to be measured according to the light emission position and the distance to the held object.
If an imaging unit for capturing the optical pattern, such as a camera, is fixed, the distance between the imaging unit and the object also changes. As a result, the imaging unit that captures an image of the object goes out of focus and fails to capture a clear optical pattern. The relative light irradiation densities occurring on the object are, therefore, not able to be measured with a high accuracy. Accordingly, the accurate optical characteristic distributions in the object, such as the optical absorption coefficient distribution, cannot be determined.